Navigation systems, such as the global positioning system (GPS) and the global navigation satellite system (GNSS) are relied upon to provide positioning information that is as accurate as possible, especially for military aircraft and weapon systems applications and for civilian aviation. Inaccurate positioning information can result in less accurate navigation-guided weapons which may increase collateral damage as well as a need for an increased number of weapons needed to accomplish a given mission. Additionally, civilian aviation applications rely on GPS/GNSS positioning information to be highly accurate for aircraft navigation.
A next generation or version of GPS that is being developed is GPS III which has a higher accuracy requirement, yet includes design considerations that may affect this higher accuracy requirement. In particular, GPS III is implemented with a steerable spot beam antenna (SBA) that introduces sources of error not previously considered. Accordingly, the design of GPS III needs to take into consideration these error sources for the higher accuracy requirement of the GPS III system.
The signals transmitted by GPS/GNSS satellites are expected or assumed to travel at exactly the speed of light by GPS/GNSS-enabled receivers employed by users of the navigation positioning systems. A GPS/GNSS receiver can receive signals transmitted by a GPS/GNSS satellite and can then measure the time that it takes for the signals to be generated and travel from the satellite to the receiver. Signal travel times multiplied by the speed of light should yield the distance between the satellite and the receiver, and these distances, called pseudo-ranges and determined for signals from at least four satellites, are then used by the receiver to compute the exact position of the mobile receiver.
Clearly any additional time or delay of a GPS/GNSS code signal being generated in a GPS/GNSS satellite will introduce a positioning error when the signal travel time is determined at a receiver. The signal receiving device only measures the signal travel time and will determine the additional time or signal generation delay as an increase in the distance between the satellite and the receiver, referred to as a ranging error. These types of timing delays are collectively referred to as a group delay which can be a culmination of various timing delays introduced into the system by a variety of sources. For example, the timing delays can be a result of any one or combination of different hardware implementations, different lengths of cabling within a satellite, antenna differences, thermal and orbital variations of the satellite as well as other environmental effects, and variations in manufacturing and calibrations which can all contribute to group delay.
A GPS/GNSS satellite can broadcast many different codes and frequencies, such as the Earth coverage GPS code signals: L1C/A, L1C, L2C, L1M, L2M, L1P, L2P and L5; and the spot beam GPS III code signals: L1M and L2M. The group delay for the different GPS/GNSS code signals can be different due to different signal paths in a GPS/GNSS satellite and other effects. For example, the different GPS III code signals L1M and L2M have different cumulative or group delays, and these differences between L1M and L2M are referred to as a differential group delay. The current GPS system uses a complex system to estimate the differential group delay between Earth coverage GPS code signals L1 and L2, for example. A ground station estimates the location of a satellite and transmits or uploads the estimate to the satellite which then provides the location estimate to GPS-enabled receivers to compensate for the L1/L2 differential group delay to within some error.
The newly developed spot beam GPS III code signals L1M and L2M can not be corrected, or compensated for, with the conventional and complex GPS technique. Conventional GPS can compensate for the L1/L2 differential group delay because the L1/L2 signals are Earth coverage GPS code signals that are available at all times to one or more ground-based GPS monitor stations. To the contrary, the spot beam antenna GPS III code signals are only temporarily available to the monitor stations and, as such, no reliable measurements of the M-code signal broadcast by a spot beam antenna can be determined. A spot beam antenna transmission is not wide enough to continuously cover at least one of the several GPS monitor stations located throughout the Earth. Because the existing technique to compensate for group delay relies on receiving and monitoring the GPS satellite signals at all times, the existing technique can not be used for estimating differential group delay of GPS III code signals from a spot beam antenna.